The present invention relates to providing instruments for the alignment of an acetabular cup within the acetabulum, which may include preoperative imaging to create a patient-specific guide for predefining the resection of the acetabulum to receive the acetabular cup.
The success of hip joint replacement surgery, which is a well-accepted treatment for arthritic conditions of the hip, depends upon preoperative planning and also the proper intraoperative placement of reamers, alignment instruments, and implants, for example, in order for the function of the joint to be optimized biologically and biomechanically. In particular, in hip replacement surgery, successful hip reconstruction is predicated upon restoring the biomechanics of the hip to “normal,” as well as selecting implants of appropriate size to avoid intraoperative or postoperative complications and to ensure long-lasting function.
The hip is a ball and socket joint. In the normal hip, the femoral head is generally circular and rotates within the acetabulum which is also generally circular. Ideally, the stress transfer of body weight across the hip joint is distributed across the surface area of the femoral head and acetabulum. A distribution of stress generally results in lower stresses in the joint, as the maximum amount of surface area is being used to distribute the stress.
In the diseased hip, the ball and socket may be malformed, and may result in an abnormally uneven distribution of stress. A deformed femoral head, for example, one that is generally more oblong than circular, will transfer stress from the femoral head to the acetabulum along the periphery of the femoral head to the periphery of the acetabulum. This transfers the entire amount of stress, imported by the body weight, to a much reduced surface area, thereby increasing the stress per unit area. The resulting increased stress per unit area generally damages the joint by damaging the articular cartilage, which, may wear out.
From a biomechanical standpoint, successful hip function depends on proper orientation of the muscles in relation to the center of rotation of the joint, such that leg length and offset are equalized following surgery. Surgery may be performed to restore the length of the leg to its original length, which in turn aims to restore the original biomechanics of the joint and thereby optimize function. A preoperative determination of the precise amount of the leg-length discrepancy, if any, is necessary to correct the discrepancy intraoperatively and achieve leg-length equality, in other words, avoid over-lengthening or under-lengthening of the leg.
In addition, restoration of hip function by performance of hip replacement surgery depends upon reproduction of the femoral offset. The femoral offset is the distance from the center of rotation of the hip joint to the longitudinal axis of the femoral shaft. The accurate determination of the femoral offset is important, because the femoral offset determines the moment arm of the abductor muscles, in other words, how hard the muscles have to work. Therefore, in a surgical hip replacement procedure, the offset needs to be restored appropriately for the hip to function properly. If a hip prosthesis is installed with insufficient offset because an acetabular component, for example, an acetabular cup, has not been properly positioned in the acetabulum, the hip muscles will have to generate increased force, which may lead to discomfort and easy fatigability.
Alignment of an acetabular cup can be achieved with an alignment guide that attaches to an insertion rod for facilitating the insertion of the acetabular cup into the acetabulum. The alignment guide preferably references the surgical table on which the patient rests. Conventionally, it is assumed that the patient's pelvis is parallel to the table, and that the surgical table is parallel to the floor. Based on such assumptions, the ordinary position (in most patients) for the acetabular cup is 45° of inclination and 20° of anteversion. For a discussion of angles of anteversion and also inclination or abduction of the acetabular cup when installed in the acetabulum, see, for example, U.S. Pat. No. 6,395,005, which is incorporated by reference herein in its entirety and is fully set forth herein.
It is has been found based on post-operative x-rays, however, that despite the alignment guide being parallel to the floor during insertion of the acetabular cup, the resultant inclination or anteversion of the acetabulum in relation to the alignment guide is often different than expected and, thus, the acetabular cup has been installed at a less than ideal position. In some circumstances, the inaccurate positioning of the acetabular cup may be caused by tilting of the pelvis of the patient in the lateral decubitus position during the surgical procedure, which titling is not recognized during the procedure.
Some surgeons use intraoperative x-rays and navigation for detecting pelvic tilt intraoperatively. Intraoperative x-ray, however, is often time consuming and can potentially increase the risk of infection due to the introduction of x-ray equipment into the operating theater. The X-ray image, which is taken through the anterior/posterior (AP) view of the pelvis, typically is of poor quality and bony landmarks are often obscured, which makes accurate measurement of the pelvic tilt difficult. Also, although intraoperative x-rays may used to determine pelvic tilt, which in turn allows for a determination of the proper inclination of the acetabular component, the x-rays cannot provide information from which the proper anteversion of the acetabular component can be determined.
In light of the above, there remains a need for a straightforward method and system for the precise positioning of components relative to the acetabulum.